1. Field of the Invention
Devices, systems, and methods consistent with the invention relate to a method and apparatus of improving the weld performance of self-shielding electrodes.
2. Description of the Related Art
Since their introduction to welding, the use of self-shielding electrodes has grown rapidly. Self-shielding electrodes are used in many types of different welding operations, such as shielded metal arc welding (“SMAW”) and flux-cored arc welding (“FCAW”). The many advantages of these welding methodologies are well known and will not be repeated herein. In each of these welding methodologies the electrode generates its own shielding gas, via the flux, to remove oxygen and nitrogen from the area of the molten weld pool and to “kill” the weld pool. The shielding gas is generated by compounds contained in the welding flux which decompose during welding. The released gas reduces the partial pressure of nitrogen and oxygen in the welding arc environment so that absorption of nitrogen and oxygen from the weld pool is reduced.
The need to remove oxygen and nitrogen from the molten weld pool and “kill” the weld pool, and the reasons therefore, are also well known.
To achieve the removal of oxygen and nitrogen from the weld metal, typical self-shielding electrodes contain a certain quantity of aluminum in either the flux or the metal electrode, or both. The presence of aluminum aids in blocking nitrogen and oxygen from the weld metal, as well as “killing” the weld pool. Specifically, the added aluminum helps to take nitrogen and oxygen out of the weld pool by reacting with it to create aluminum nitride and aluminum oxide. Most of the aluminum nitride and aluminum oxide then floats out of the weld pool while some remains within the weld metal. However, the use of aluminum is not without its drawbacks.
Specifically, the presence of aluminum in the weld metal has the tendency to close the gamma loop on the iron-carbon phase diagram, see FIG. 1. Because of this, the presence of aluminum tends to restrict the phase transformation from the delta to the gamma to the alpha phases. A result of this restriction is the creation of large unrefined grains in the structure of the weld metal. The presence of large unrefined grain structure in the weld metal results in a weld which has poor ductility, i.e. the weld is brittle. In many applications, a brittle weld is undesirable.
These drawbacks from aluminum can be enhanced due to the use of lithium ferrate in the electrodes. Lithium ferrate is often used in self-shielding electrodes because the lithium disassociates to lithium gas which aids in diluting the nitrogen from air around the weld. However, because of the intense heat and energy which is typical to welding the lithium ferrate may react with the aluminum from the electrode in what is known as a thermite reaction. Because of this reaction it is often necessary to add additional aluminum to the electrode. However, often the thermite reaction goes to varying degrees of completion. Because of this, any excess aluminum which is not reacted will end up in the weld metal in varying concentrations, often within the same weld. This is disadvantageous for the reasons set forth above.
Accordingly, an electrode composition is needed which blocks the entry of nitrogen and oxygen into the weld metal and does not close or significantly interfere with the phase transformations in the weld metal.